346 



AGRICULTURAL ENGINEERING 



engine is the more simple to explain of the two types and, for 

 that reason, should be considered first. It is to be assumed 

 that the reader understands the gas engine to consist of the 

 essential parts as illustrated in Fig. 221. These parts, as 

 far as a consideration of the cycles is concerned, consist of a 

 cylinder with a gas-tight piston attached by a connecting rod 

 to a crank on which the fly wheels and pulleys are attached, 

 and two valves, an inlet valve to let the gases into the 



Fig. 221. 



Illustrating the operations which take place in a four-stroke 

 cycle engine to obtain a power or working stroke. 



cylinder and an exhaust valve to let the burnt gases out. 



Four-Stroke Cycle. The four strokes in the four-stroke 

 cycle engine are: (SeeFig. 221.) 



First, the suction stroke, during which the piston increases 

 the volume of the space at the closed end of the cylinder and 

 thus draws into the cylinder through the inlet valve a charge 

 of vaporized fuel, and enough air to furnish a sufficient 

 amount of oxygen to support combustion. 



Second, the compression stroke, during which the piston 

 makes a return stroke and compresses the gases into the clear- 



